In general, a solar cell is formed using a semiconductor wafer including a silicon wafer. In order to improve conversion efficiency of the solar cell, it is necessary to reduce the light that reflects on a light receiving surface of the solar cell and the light that passes through the solar cell. Here, for example, in preparing a crystalline solar cell by using the silicon wafer, since the silicon wafer has a low transmittance of visible light contributing to photoelectric conversion, in order to improve the conversion efficiency, it is necessary to consider only reducing reflection loss of the visible light on the silicon wafer surface acting as the light receiving surface, thereby effectively capturing incident light within the solar cell.
As a technique for reducing the reflection loss of the incident light on the silicon wafer surface, there are a technique to form an antireflection film on the surface and a technique to form, in the surface, an uneven structure such as micro-sized pyramidal unevenness called a textured structure. As for the latter technique, the method of forming the textured structure in the surface is suitable for single crystal silicon and represented by a method of etching a surface (100) of single crystal silicon with alkali solution. The etching using alkali leverages that an etching rate of a surface (111) is slower than that of the surface (100) and a surface (110). Further, as the latter technique, in recent years there is suggested a method, by making a silicon surface porous, of forming the uneven structure in the surface to reduce the reflection loss of the incident light.
For example, Patent Document 1 describes a method, by an anodization process that passes an electric current in hydrofluoric acid using a single crystal silicon substrate as an anode and Pt as a cathode, of forming numerous fine holes in the surface.
Further, Patent Document 2 describes a technique, in order to form finer sub-micron order unevenness on micron-sized texture structure formed in a surface of a silicon substrate, of carrying out electroless plating of the surface with metal particles and then etching the substrate in mixed solution of an oxidizing agent and hydrofluoric acid. Specifically, an alkali textured p-type single crystal silicon substrate is immersed in solution containing silver perchlorate and sodium hydroxide, such that silver particles are formed on the surface. Then, the silicon substrate is immersed in mixed solution of hydrogen peroxide water, hydrofluoric acid and the water, to form sub-micron order unevenness.